docs: def edge_style(self, head, tail, **kwargs):
          '''
          Modifies an edge style to the dot representation.
          '''
          if tail not in self.nodes:
              raise GraphError("invalid node %s" % (tail,))
  
          try:
              if tail not in self.edges[head]:
                  self.edges[head][tail]= {}
              self.edges[head][tail] = kwargs
          except KeyError:
              raise GraphError("invalid edge  %s -> %s " % (head, tail) )